runtime/protoimpl: remove ExtensionFieldsV1

This is no longer directly used by v1.

Change-Id: I14a283dd2d1572d50ebd82fdf55f7d86bfacc272
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/172438
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/legacy_test.go b/internal/impl/legacy_test.go
index 1bc5acf..84cf071 100644
--- a/internal/impl/legacy_test.go
+++ b/internal/impl/legacy_test.go
@@ -29,7 +29,7 @@
 
 type legacyTestMessage struct {
 	XXX_unrecognized       []byte
-	XXX_InternalExtensions pimpl.ExtensionFieldsV1
+	XXX_InternalExtensions map[int32]pimpl.ExtensionFieldV1
 }
 
 func (*legacyTestMessage) Reset()         {}